Various manufacturing methods for glass optical elements have been known in which a glass preform as a glass material to be molded is press-molded with a mold guaranteeing surface precision and surface roughness necessary for molded glass article surface where grinding or polishing after press-molding is made unnecessary.
For example, methods as set forth in Japanese Unexamined Patent Publication, No. Showa 64-72,929 or Japanese Patent Publication, No. Heisei 2-16,251 are methods in which molds and glass preforms are heated together. That is, a glass preform is inserted in a mold constituted of an upper mold, a lower mold, and guide molds for guiding them and is press-molded after heated with the mold up to a temperature that the glass preform is adequately softened. The molded preform is then cooled down to about the glass transition point with a cooling rate such that the surface precision of the molded glass article at the post molding stage would not be impaired (or cooled down to around a room temperature in spending a certain time subsequently), and thereafter, the molded glass article is removed from the mold.
In a method, as described above, in which a glass preform is heated, molded, and cooled together with the mold while the glass preform is held in the mold, the temperatures of the glass and the mold are always almost equal to each other, and as the pressing process proceeds, the temperature difference between the surface and inside of the glass is minimized, thereby hardly producing sink marks and thereby offering a stable surface precision. However, this method has a demerit that the cycle time for the entire processes becomes considerably longer because it requires a period for increasing temperature before the pressing starts and a period for decreasing temperature before the article is removed after pressing. Moreover, since the glass and the mold surface contact to each other for a long time from the heating process to the pressing process, reactions tend to occur between the glass and the mold, thereby shortening the life period of the mold.
A method has been known in which a glass preform softened in advance is inserted in a mold separately heated and can renders grinding and polishing after press-molding unnecessary (e.g., Japanese Unexamined Patent Publication No. Showa 61-251,529, Japanese Unexamined Patent Publication No. Showa 61-286,232, Japanese Unexamined Patent Publication No. Showa 62-27,334, Japanese Unexamined Patent Publication No. Showa 63-45,134). With this method, also used is a mold guaranteeing surface precision and surface roughness necessary for molded glass article surface.
The method in which a glass preform softened by heat is press-molded with a preheated mold has an advantage that the method can make the press time shorter. The method also can make the temperature of the mold relatively lower, and because mold releasing is possible after a certain time passes after pressing for cooling the glass temperature, the method can largely shorten the cycle time.
However, this method could not mold the glass as to have a desired shape when the glass material is required to be transformed largely. That is, this method normally uses a mold held at a relatively lower temperature to shorten the cycle time. The glass material may therefore be solidified when cooled, before the glass material is press-molded to have a desired thickness. As a result, this method raised a problem that some molded glass articles obtained by largely transforming the glass material such as, particularly, bi-convex lenses having a thin edge thickness (about 0.8 to 1.3 mm), meniscus lenses, and concave lenses could not be obtained stably. This method also raised a similar problem caused by a large amount of transformation where a spherical glass material is molded. This method also had disadvantages that sink marks or distortions in a material shape may occur easily and satisfactory surface precision is hardly obtainable. In other words, there was a problem that configuration transferability is inadequate.
Particularly, when the glass preform is manufactured by a hot-molding method, the glass preform normally has a shape of a sphere or a slightly flatted sphere or echinus shape, which is not close to the final products. To transform such a glass preform to a desired shape, because the glass preform needs to be largely transformed, such a glass preform also raised the above problem.
It is an object of the invention to provide a method for manufacturing glass optical elements, which can mold the elements in a relatively short cycle time nearly the same as that of conventional methods, can produce stably glass molded articles with good surface precision, even where the glass optical elements are manufactured by largely transforming a glass material in a case such that a glass material having a shape largely different from the shape of the final product is used.
The inventors diligently conducted researches to solve above-described problems in the prior art. They consequently have discovered that, when press-molding of glass optical elements is made, the glass optical elements can be manufactured with good surface precision where the cycle time of the press-molding process is maintained in a short period, by press-fabrication upon preheating the glass material in keeping the temperature (viscosity) of the glass material and the temperature of the mold to have a constant relation. The inventors further perfected the invention upon finding that a glass molded article is obtainable with very good surface precision by pressing the material until the center thickness becomes 70% or less where the glass material is press-molded under the above temperature condition.